image: ©jadamprostore | iStock ISRO scientists have discovered a “transition boundary” that causes space junk to fall to Earth significantly faster once solar activity reaches 67% of its peak.
By using 60-year-old debris as atmospheric probes, the team has provided a new formula for predicting orbital decay, helping satellite operators save fuel and avoid collisions during periods of high solar activity
A study led by the Vikram Sarabhai Space Centre (ISRO), published in Frontiers in Astronomy and Space Sciences has identified a predictable “transition boundary” for space junk.
For the first time, scientists can quantify exactly when solar activity shifts from a background influence to a powerful driver of orbital decay, potentially saving satellites from catastrophic collisions.
The thermospheric “drag” effect
Low Earth Orbit (LEO) is currently a crowded “highway” for mega-constellations like Starlink and surveillance satellites, but it is also littered with decades of debris. The Sun acts as a natural cleaner for this region through a cycle of heating.
During peak activity, the Sun emits intense UV radiation and charged particles that heat Earth’s thermosphere, causing it to expand upward. This expansion increases the atmospheric density around orbiting bodies, creating a “drag” or resistance that slows them down and causes them to fall and burn up in the atmosphere much faster.
The 67% tipping point for space junk
By tracking the trajectories of 17 pieces of “dead” space junk launched in the 1960s, researchers discovered that there is a specific threshold where this process accelerates.
The transition boundary:
When solar activity (measured by sunspot numbers) reaches approximately two-thirds (67%) of its peak, the rate at which space junk loses altitude increases dramatically.
Non-linear decay:
Below this threshold, decay is slow and steady. Once the Sun passes this point, the atmosphere becomes dense enough to “pull” objects down at a significantly faster pace.
Better fuel planning for satellites
This discovery is a critical tool for active missions. Because active satellites are affected by the same drag as space junk, operators can now better predict how long their fleets will last.
Satellites launched near a solar maximum will require significantly more fuel for “station-keeping” to fight the increased drag. Furthermore, knowing exactly when the “cleansing” effect kicks in allows for more accurate tracking of drifting debris, helping to prevent the “domino effect” of orbital collisions.